Alumina ceramic substrates are widely used as substrates having excellent electrical, mechanical and chemical properties in the microelectronics industry. In addition to these properties, in the case of a substrate for a thin film hybrid integrated circuit, the surface smoothness thereof is one of the important properties. Furthermore, in the case of a substrate for a microwave application, the dielectric of the dielectric constant .epsilon. and the dielectric loss tangent tan .delta. are important. Therefore, there has been a great demand for an alumina substrate having a high degree of surface smoothness and excellent dielectric properties.
An alumina substrate having the above-mentioned properties may be manufactured by using a fine and highly pure alumina powder, which must be produced by a process other than the known Bayer process, by adding magnesium oxide (MgO) and chromic oxide (Cr.sub.2 O.sub.3) into the powder, and then sintering the powder (as described in the example in the specification of U.S. Pat. No. 3,854,965). However, the price of this pure alumina powder is ten or more times as high as that of an alumina powder produced by the widely used Bayer process.
The inventors have filed another patent application (which issued as U.S. Pat. No. 4,045,412) disclosing a process for manufacturing an alumina substrate having a high density and a high degree of surface smoothness. The properties are attained by increasing the density of a green sheet formed from alumina powder produced by the Bayer process. The surface smoothness and the sintered density of the alumina substrate manufactured by the proposed process were almost equal to those of an alumina substrate manufactured by the process in the above identified U.S. Pat. No. 3,854,965. However, the proposed process failed to improve the dielectric properties sufficiently. Thus, the dielectric properties of the alumina substrate manufactured by the proposed process were lower than those of the substrate manufactured by the process in the above U.S. Pat. No. 3,854,965.
Thereafter, the inventors continued their study with regard to alumina ceramic substrates and found that the surface smoothness and dielectric properties of a substrate made from the Bayer process alumina powder are influenced by a hydrogen atmosphere condition, e.g. contamination of the atmosphere in a firing process, and by the kinds and quantities of impurities contained in the alumina powder that is used. Even if alumina powders have the same purity, there are generally differences in the kinds and quantities of the impurities contained in the powders.